This invention relates, in general, to a laser diode matching circuit and is particularly, but not exclusively applicable to impedance matching in fibre optic transceivers in which the laser diode is packaged in a transistor outline (TO) package.
A conventional fibre optic transceiver module contains a transmitter section and a receiver section. The transmitter section normally comprises a laser diode in a package (such as a TO-can having suitably embedded and extended electrical contact leads) that is driven by an integrated circuit or a set of discrete components that form the so-called xe2x80x9cLaser Diode Driverxe2x80x9d. The laser diode driver drives current through the laser diode to produce pulses of light (out of the laser diode) at given time intervals dependent on the input signal applied at the input of the laser diode driver.
As driving, i.e. operating, speeds increase into the gigahertz (GHz) region, it becomes increasingly important to match electrically the output impedance of the laser diode driver to the input impedance of the laser diode. A poor electrical match will generally result in electrical reflections (through the electrical leads), parasitic resonance caused by stray capacitances and inductances and general noise. Failure to provide a clean drive signal therefore ultimately causes a poor quality optical signal that could contain degradations in amplitude and time. Reflections through leads and tracks (resulting from impedance mismatches) cause ripples or oscillations on a time domain detection signal. More especially, parasitic resonance can lead to deformation of a detection eye (of the time domain signal) that is used to differentiate between binary logic levels (i.e. lasing and dark operation).
Unfortunately, the construction of laser diode packages inherently leads to the introduction of parasitic resonance since ingress and egress of electrical leads into the TO-can are typically through a glass-resin insulating sleeve set within a metal plate, e.g. made of mild steel. With the electrical leads, in operation, carrying current, the glass-resin (or equivalent) insulating sleeve acts as a dielectric and forms a capacitor that resonates at certain fibre operating frequencies. Also, while the length of the electrical leads may be optimised to prevent electrical reflections, the inexorable variation in nominal circuit component values results in an output from the laser diode driver being load/impedance circuit specific; this means that tuning of individual circuits for optimised operation is required.
Generally, in order to effectively improve laser driver operation it is necessary to kill the parasitic resonances and/or reflections; this can be achieved by using a parallel dummy load to present an alternative path to earth potential (in the case of a filter) or by restricting parasitic resonances and reflections by improving impedance matching.
Matching of a laser diode to a laser diode driver is presently accomplished through use a simple resistor-capacitor (RC) shunt network to ground or the supply voltage, Vcc. The RC shunt network is placed between the laser diode and laser driver circuit, with the shunt providing a matched impedance at a certain frequency determined by values of the individual components of the RC shunt network. Unfortunately, the problems of using an RC shunt are that: i) the matching frequency is insufficiently broad enough to cover the entire operating frequency range required; ii) the match is tuneable only by replacing the resistor and capacitor values, i.e. the shunt is circuit specific; and iii) the shunt network is too harsh and results in loss of bandwidth. Furthermore, use of an RC network (i.e. a filter) in a laser device, typically operating at a bit rate in excess of two gigabits per second (2 Gbit/s), is compromised in terms of a clean detection window (in the time domain eye detection signal). Clearly, at laser operating frequencies and correspondingly high data rates, it is necessary for a steady state signal condition to be met quickly in order to maintain integrity in the detection algorithm that is operational on the time domain eye signal. However, the RC shunt achieves the requisite damping effect in logic level signal transitions (viewed in the time domain) when employing a large time constant that is incompatible with the relatively short rise and fall time allotted to data level stabilisation at laser operating frequencies.
According to a first aspect of the present invention there is provided a method of impedance matching a laser driver with a laser diode coupled thereto, the method characterised by: coupling a common node located between the laser driver and the laser diode to a reference potential through a matching stub and an AC shorting capacitor; and tapping the matching stub with the AC shorting capacitor to provide an impedance match between the laser driver (20) and the laser diode.
In another aspect of the present invention there is provided a semiconductor laser control circuit comprising a laser driver integrated circuit, the semiconductor laser control circuit characterised by: at least one microwave transmission line connectable to the laser driver integrated circuit, the microwave transmission line further coupled to a reference potential plane of the semiconductor laser control circuit through an AC shorting capacitor or a zero Ohm link to provide, in use, a matching impedance between the laser driver integrated circuit and a semiconductor laser.
In yet another aspect of the present invention there is provided a semiconductor laser arrangement comprising a TO-can having a laser diode and a laser driver control assembly operable to control the laser diode, characterised in that: the laser driver control assembly includes a microwave transmission line connected to a laser driver integrated circuit through a stub resistor, the microwave transmission line further coupled to a reference potential plane through an AC shorting capacitor that taps the microwave transmission line to provide impedance matching between the laser driver integrated circuit and the TO-can.
Preferably, a stub-matching element of the present invention is created using a microstrip line on a PCB, the microstrip line having a characteristic impedance Z0. The microstrip line is connected at a node between the laser diode and laser diode driver devices at some (preferably optimum) distance away from the laser diode. If the Q-factor of the stub is to be reduced, the microstrip line can be connected via a series resistor, Rstub; this allows for a broader matching effect without changing the position of a stub capacitor Cstub. If an ac short is required, the microstrip line is terminated to a suitable point (nominally Ground) via a capacitor, Cstub. If a DC/AC short is required, the microstrip is simply connected to the reference point by a zero ohm (0 xcexa9) link.
A plurality of stub-matching elements of differing lengths may be provided on a single PCB.
Advantageously, the present invention provides a mechanism for improving impedance matching in laser diode driver circuitry, the mechanism making use of physical properties intrinsic to the operating medium to control electrical effects. The present invention is inexpensive to implement in a laser diode driver IC (or on an associated printed circuit board, PCB), with the circuit of the present invention readily adaptable to different TO-cans or the like. In other words, the present invention removes the constraints on impedance matching from a circuit specific arrangement to a general case solution. Using stub-matching and, more generally, distributed elements to match a laser diode to a laser diode driver provides a more accurate and versatile method of impedance matching than the normal RC (filter) shunt matching method.
Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which:
FIG. 1 is a schematic block diagram of a laser diode driver circuit and associated TO-can that can support the concepts of the present invention;
FIG. 2 is a conventional eye diagram showing time domain representations of data level transitions;
FIG. 3 is a circuit diagram of an impedance stub-matching circuit according to a preferred embodiment of the present invention;
FIG. 4 is an alternative embodiment of an impedance stub-matching circuit according to the present invention; and
FIG. 5 is another embodiment of an impedance stub-matching circuit according to the present invention